1. Field of the Invention
Ultrasonic probe heads of this kind have been known for a long time and are used, for example, in the field of process engineering in ultrasonic flow meters, wherein, in this case, the use of the ultrasonic probe head is not of importance, and the subsequently described teaching of the invention can, moreover, be used by a plurality of different ultrasonic probe heads.
In the ultrasonic probe head in question, a preferably good connection between the ultrasonic transducer and the ultrasonic window of the housing is decisive, wherein “a preferably good” connection means that the contact surface formed, on the one hand, by the surface of the ultrasonic transducer and, on the other hand, by the surface of the ultrasonic window is preferably large so that the ultrasonic signals emitted by the ultrasonic transducer or, respectively to be received, can pass, preferably unobstructed, over the previously-mentioned contact surface. When the contact surface is only inadequately formed and air pockets exist between the ultrasonic transducer and the ultrasonic window, the acoustic interface between the ultrasonic transducer and the ultrasonic window is limited, since the ultrasonic signals are practically reflected at the boundary layer to air and, thus, cannot conversely exchange from the ultrasonic transducer to the ultrasonic window.
2. The Prior Art
It is known from the prior art to glue the ultrasonic transducer and the ultrasonic window to one another with an adhesive. The problem with this is that the known and used adhesives often have only a marginal resistance to heat exposure and, among other things, gasify under heat exposure with the result that gas pockets are formed within the adhesive having undesired boundary layers; this implicates the negative effects for the transmission ability of the ultrasonic signals described above. Additionally, it is disadvantageous that the ultrasonic transducer can disconnect entirely from the ultrasonic window or, respectively, the housing due to the diminished stability of the adhesive under heat exposure. Furthermore, it has been seen that adhesives also lose their effectiveness at very low temperatures, as e.g. in so-called cryogenic applications, so that ultrasonic probe heads having a glued ultrasonic transducer are presently only able to be used in a limited range of temperatures from about −40° C. to about +150° C.
It is also known from the prior art to make a moist interface between the ultrasonic transducer and the ultrasonic window of the housing, for example by using liquids having an oil base. The same occurrences apply for high and low temperature applications with such moist interfaces as in the above-mentioned glued interface. The liquids used are unstable at high temperatures, e.g. they may gasify, and lose their connecting effectiveness at very low temperatures.
It is known from DE 100 21 187 C1 to make an ultrasonic probe head usable for high and low temperature applications by thermally insulating the ultrasonic transducer using an ultrasonic waveguide and thus also the interface from the ultrasonic transducer and ultrasonic waveguide. However, using an ultrasonic waveguide is disadvantageous in that an additional signal propagation delay has to be taken into consideration, which entails an additional uncertainty of measurement. In particular, in low temperature applications, an undesired heat input in the measuring media can occur via the ultrasonic waveguide which may lead to a change of the media that can seriously affect measurement. For example, contact with liquid nitrogen, which has a boiling point of about −196° C. at atmospheric pressure, can quickly lead to the formation of bubbles on the end of the ultrasonic window or, respectively the ultrasonic waveguide facing the media, which, once again, substantially limits transmission of the ultrasonic signal.